This mini-lesson walks you through the whole of Edexcel Topic 9 — Separate chemistry 2: qualitative analysis (tests for ions and gases), instrumental methods, nanoparticles, hydrocarbons, alcohols & carboxylic acids, and polymers.
Work through each screen, answer the questions as you go (some recall, some predicting reactions) and collect ⭐ stars. The purple HT tag marks Higher-tier-only content. Press Start when you're ready.
Tests for cations · flame tests
Flame tests for metal ions
Some metal ions give a characteristic flame colour. Dip a clean wire in the solid, then hold it in a hot blue Bunsen flame and read off the colour:
Use a clean wire each time — a trace of sodium gives a strong yellow that masks other colours.
Watch out: the test for an ion must be unique — if two ions gave the same result you couldn't tell them apart. Sodium's strong yellow can hide a weaker colour, which is why the wire must be clean.
Quick check
Read the flame
?A solid is held in a hot Bunsen flame and the flame turns lilac. Which metal ion is present?
Match it
Ion → flame colour
Tap an ion on the left, then its flame colour on the right.
Tests for cations · NaOH precipitates
Hydroxide precipitate colours
Add a few drops of sodium hydroxide solution to a solution of the salt. Many metal ions form an insoluble metal hydroxide — a coloured solid (precipitate) you can identify:
Three ions give a white precipitate — only Al³⁺ re-dissolves to a colourless solution when you add excess NaOH.
Watch out: Cu²⁺ blue, Fe²⁺ green and Fe³⁺ brown are easy to mix up. Al³⁺, Ca²⁺ and Mg²⁺ all give white — to tell Al³⁺ apart, add more NaOH: its precipitate dissolves; the others stay solid.
Quick check
Name the ion
?Adding NaOH(aq) to a solution gives a green precipitate. Which ion is present?
Sulfate, SO₄²⁻: add dilute hydrochloric acid then barium chloride solution → a white precipitate (barium sulfate).
Halides (Cl⁻, Br⁻, I⁻): add dilute nitric acid then silver nitrate solution → a coloured silver-halide precipitate.
Silver chloride white → silver bromide cream → silver iodide yellow. The colour deepens down the group.
Why the acid first? The dilute acid removes carbonate ions that would otherwise also give a precipitate and a false positive.
Quick check
Which anion?
?A solution is acidified with dilute nitric acid, then silver nitrate is added. A cream precipitate forms. Which ion is present?
Ammonium ion & tests for gases
Ammonium & the gas tests
Ammonium ion, NH₄⁺: add NaOH and warm gently → ammonia gas is given off, which turns damp red litmus paper blue.
You also need the five gas tests:
Two gases turn damp litmus: Cl₂ bleaches it (goes white), NH₃ turns red litmus blue (it is alkaline).
Watch out: hydrogen gives the pop, oxygen relights the glowing splint — don't swap them. Chlorine bleaches litmus; ammonia turns red litmus blue.
Sort it
Name the gas
Tap the gas that matches each test result.
Instrumental methods
Flame photometry
Instrumental methods use machines instead of test-tube chemistry. They improve sensitivity (detect tiny amounts), accuracy and speed.
A flame photometer can identify metal ions and measure their concentration. You build a calibration curve from solutions of known concentration, then read your unknown off the line:
Read the unknown's signal across to the line, then down to find its concentration. Identify the metal by comparing its pattern with reference data.
You don't need to know how the instrument works — just that it is more sensitive, accurate and faster than the test-tube tests, and how to read a calibration curve.
Bulk & surface properties
Nanoparticles
Nanoparticles are tiny — roughly 1 to 100 nm across — only a few hundred atoms wide, far smaller than the particles in fine powders.
As particles get smaller, their surface-area-to-volume ratio shoots up. A huge fraction of the atoms sit on the surface, so nanoparticles can be very reactive and effective as catalysts, and you need far less material:
Same total volume, far more surface. Uses: sunscreens, catalysts, antibacterial coatings, electronics. Risks: their huge reactive surface and tiny size mean possible unknown effects on health and the environment.
Watch out: as a cube's side halves, its volume falls faster than its surface area — so the surface-area-to-volume ratio gets bigger. That high SA:V is the whole point of nanoparticles.
Quick check
Why nano?
?Compared with the same mass of a normal powder, why can a nanoparticle catalyst work with far less material?
Hydrocarbons
Alkanes vs alkenes
A hydrocarbon contains only hydrogen and carbon. Two families matter here:
Alkanes — general formula CnH2n+2. Only single C–C bonds, so they are saturated (methane CH₄, ethane C₂H₆, propane C₃H₈, butane C₄H₁₀).
Alkenes — general formula CnH2n. Contain a C=C double bond (the functional group), so they are unsaturated (ethene C₂H₄, propene C₃H₆, butene C₄H₈).
Alkane = all single bonds (saturated). Alkene = one C=C double bond (unsaturated).
Both burn — complete combustion oxidises the hydrocarbon to carbon dioxide + water. Cracking breaks long alkanes into smaller, more useful alkanes and alkenes.
Hydrocarbons · the bromine test
The bromine water test
The C=C double bond lets alkenes do addition reactions. Shake a hydrocarbon with orange bromine water:
Alkene + bromine water → the orange colour disappears (e.g. ethene C₂H₄ + Br₂ → dibromoethane C₂H₄Br₂). An alkane leaves it orange.
Watch out: "decolourise" means the orange disappears — it does not go white or cloudy. Only the unsaturated alkene decolourises bromine water; the saturated alkane has no C=C to react.
Quick check
Predict the test
?A gas is shaken with orange bromine water and the colour quickly disappears. What does this tell you about the gas?
Sort it
Alkane or alkene?
Tap a molecule, then tap the family it belongs in. (Alkanes CnH2n+2, alkenes CnH2n.)
🔗 Alkane (saturated)
🟰 Alkene (unsaturated)
Alcohols
Alcohols & ethanol
Alcohols all contain the –OH functional group: methanol CH₃OH, ethanol C₂H₅OH, propanol, butanol. Ethanol can be made two ways:
Fermentation of sugars (carbohydrates) in solution, using yeast (which provides enzymes), around 30–40 °C without air: sugar → ethanol + carbon dioxide. A concentrated solution is then obtained by fractional distillation.
Hydration of ethene with steam and a catalyst: ethene + steam → ethanol (a continuous, faster industrial route from crude oil).
Alcohols burn (combust) releasing energy, and ethanol can be oxidised to ethanoic acid (vinegar) — the same happens when wine goes sour in air.
Fermentation vs hydration: fermentation uses a renewable feedstock (sugar) but is slow and batch; hydration of ethene is fast and continuous but uses a finite resource (crude oil).
Quick check
Making ethanol
?Yeast is added to a sugar solution and kept warm with no air. Ethanol forms. What is this process called?
Carboxylic acids
Carboxylic acids
Carboxylic acids contain the –COOH functional group: methanoic, ethanoic (CH₃COOH), propanoic, butanoic acids. Ethanoic acid is what makes vinegar sour.
They are weak acids — in water only a small fraction of their molecules ionise (split into ions), so for the same concentration they have a higher pH (less acidic) than a strong acid like hydrochloric acid.
Like other acids they react with carbonates to give a salt + water + carbon dioxide, and with metals to give a salt + hydrogen. Members of a homologous series react similarly because they share the same functional group.
Weak ≠ dilute. "Weak" means only partly ionised; "dilute" means a low concentration. A weak acid can still be concentrated.
Quick check
Weak acids
?Ethanoic acid is described as a weak acid. What does "weak" mean here?
Polymers · addition polymerisation
Addition polymers
A polymer is a very large molecule made of many small repeating units joined together. In addition polymerisation, many alkene monomers add together — the C=C double bonds open up and link into a long chain, with no other product.
The C=C opens; the repeating unit is drawn with a bond out of each side. Other monomers give poly(propene), poly(chloroethene) (PVC) and PTFE.
Watch out: the repeating unit has single C–C bonds (the double bond has opened) and a bond sticking out of each side. Deduce the monomer by putting the C=C back.
Quick check
Monomer ↔ repeat unit
?The repeating unit of poly(propene) is –[CH(CH₃)–CH₂]–. What is the monomer it was made from?
Higher tier only
Condensation polymers
A polyester is a condensation polymer. Two different monomers join — one with two –COOH groups (a dicarboxylic acid) and one with two –OH groups (a diol):
Each time an ester link forms, a small molecule of water is released — that loss of a small molecule is why it is called condensation polymerisation.
Key difference: addition polymerisation makes only the polymer (one product); condensation polymerisation also releases a small molecule (here, water).
Higher tier · Quick check
Addition vs condensation
?What is the key difference between condensation polymerisation and addition polymerisation?
Polymers · disposal
The problem with polymers
Most addition polymers are cheap and useful, but disposing of them causes real problems:
Their starting materials come mostly from finite crude oil.
They are non-biodegradable, so they persist in landfill for a very long time.
Burning them to dispose of them produces toxic gases (e.g. CO₂, and HCl from PVC).
To recycle, polymers must first be sorted by type before they can be melted and reformed.
Recycling trade-off: recycling saves crude oil and landfill space, but sorting and reprocessing cost energy and money — so you must evaluate the economic and environmental pros and cons.
Recap
The key facts to know
Flame tests: Li red · Na yellow · K lilac · Ca orange-red · Cu blue-green
NaOH precipitates: Cu²⁺ blue · Fe²⁺ green · Fe³⁺ brown · Al³⁺/Ca²⁺/Mg²⁺ white (Al³⁺ dissolves in excess)